I recently took over support of an installation of OpenEMM and a few days ago mails started not going out anymore.
Now I have to admit I am relatively new to OpenEMM, but after days of trying things out I am simply running out of ideas. Any help or insight would be greatly appreciated.

The whole process of generating a mailing in the web frontend works flawless.
It is just that the mails are stuck in ~/var/spool/QUEUE and don't leave this queue.
And I honestly have no idea why.
The sendmail-system on this server has not been touched in ... a long time so there should not be any change.

If sendmail is enabled check the process list for running sendmail and search for an instance processing the directory /home/openemm/var/spool/QUEUE. If this is missing, the easiest way to fix this is restarting openemm.

If this still fails, check the sendmail logfile /var/log/maillog for further hints.

And, if you haven't tried already, reboot the whole server and check if the problem is still existing.

thank you so far.
According to your code, sendmail is running.
However I cannot find any sendmail processes (like at all, so it might be just me)
The only thing in the process list doiung anything with the queue is this one: